Waxes in wax-containing hydrocarbons oils are removed therefrom by chilling the oil to precipitate out the wax and then separating the solid wax particles from the dewaxed oil by solid/liquid separation procedures such as filtration, centrifugation, settling, etc. Industrial dewaxing processes include press dewaxing processes wherein the wax-containing oil, in the absence of solvent, is chilled to crystallize out the wax particles, which are then pressed out by a filter. In general, only light hydrocarbon oil fractions are treated by press dewaxing processes due to viscosity limitations. More widely used are solvent dewaxing processes wherein a waxy oil is mixed with a solvent and then chilled to precipitate the wax as tiny particles or crystals, thereby forming a slurry comprising solid wax particles and a solution of dewaxed oil containing dewaxing solvent. The slurry is then fed to a wax separator (e.g., filter) wherein the wax is removed from the homogeneous solution of dewaxed oil and dewaxing solvent. Solvent dewaxing processes are used for heavier oil fractions such as lubricating oil distillates and Bright Stocks. Typical dewaxing solvents include low boiling point, normally gaseous, autorefrigerative hydrocarbons such as propane, propylene, butane, pentane, etc., ketones such as acetone, methylethyl ketone (MEK), methylisobutyl ketone (MIBK) and mixtures thereof, aromatic hydrocarbons such as benzene, toluene and xylene, as well as mixtures of ketones and aromatic hydrocarbons such as MEK/toluene and acetone/benzene and mixtures of ketones with autorefrigerants such as acetone/propylene.
One of the factors tending to limit the capacity of a solvent dewaxing plant is the rate of wax filtration (i.e., separation in general) from the dewaxed oil, which in turn is strongly influenced by the crystal structure of the precipitated wax. Although the crystal structure of the precipitated wax is influenced by various operating conditions in the dewaxing process, for any given feed it is most strongly influenced by the chilling conditions. The size and structure of the precipitated waxy crystals, the amount of oil occluded in the wax crystal and the condition and quantity of the oil left in the crystal are extremely varied and depend on the wax composition and precipitation conditions. These conditions also affect the separation (filtration) rate of the dewaxed oil from the wax and the yield of dewaxed oil. In some cases, most notably when the waxy oil is a Bright Stock, the waxy crystals are of an extremely fine size and not all are separated by filtration, but some leave the filter with the dewaxed oil component which creates an objectionable haze in the oil.
One way of improving the filtration rate and minimizing haze formation is to add a dewaxing aid to the waxy oil during the dewaxing process. The aid may be either mixed with the waxy oil prior to chilling, or introduced during the chilling process employing either indirect chilling means, such as scraped surface chillers, or alternatively, direct chilling means employing cold solvent. Preferred direct chilling means employing cold solvent injected along a number of stages therein, a number of which stages are highly agitated during instantaneous mixing, is DILCHILL.RTM. dewaxing (registered service mark of Exxon Research and Engineering Company) a process as presented in U.S. Pat. No. 3,773,650, hereby incorporated by reference. Other preferred direct chilling means is the autorefrigerative chilling process employing liquified normally gaseous hydrocarbons like propane as solvent.